1. Field of the Invention
The invention relates to a fuel system for delivering fuel to an internal combustion engine, with a reservoir, a first fuel pump whose input side is connected to the reservoir, a second fuel pump whose input side is connected to the first fuel pump, at least one injection valve that is connected to the second fuel pump and can supply fuel at least indirectly to a combustion chamber, and a leakage line provided between the second fuel pump and the reservoir.
2. Description of the Prior Art
A fuel system of the kind described above is known from the market. In the known fuel system, a first fuel pump delivers fuel from a fuel reservoir to a second fuel pump by means of a fuel line. The second fuel pump is a high-pressure fuel pump, which delivers the fuel at a very high pressure into a fuel accumulation line (also referred to as the xe2x80x9crailxe2x80x9d). From there, the fuel travels to at least one injection valve through which the fuel finally travels into the combustion chamber.
Normally, the number of injection valves is equal to the number of cylinders in the engine. The fuel system can be designed so that the injection valve injects the fuel directly into a combustion chamber of the engine. In the known fuel system, a single cylinder piston pump is used as the high-pressure fuel pump. Leakage fuel, which passes through the gap between the cylinder and the piston, is returned from the high-pressure fuel pump to the reservoir by means of the leakage line. This eases the burden on the piston seal of the single cylinder piston pump used.
Supplying fuel to the combustion chambers of the engine during the starting process is a fundamental problem in fuel systems. In the known fuel system, a valve device assures that during the starting process, the first fuel pump supplies the fuel to the injection valves at an increased delivery pressure. In many cases, this increased delivery pressure is sufficient to start the engine in an extremely short period of time. The increased delivery pressure can in many cases compress a gas bubble possibly present in the fuel connection between the first fuel pump and the second fuel pump, thus assuring a reliable operation of the engine.
The object of the current invention is to modify a fuel system of the type mentioned at the beginning so that the starting and operating behavior of an engine that is equipped with the fuel system is further improved at high operating temperatures and the service life of the fuel system is as long as possible.
In a fuel system of the type mentioned at the beginning, this object is attained by virtue of the fact that the leakage line contains a valve device with a shutoff function and a pressure relief function that are connected in parallel with each other.
Providing a valve device with a shutoff function in the leakage line maintains the increased initial pressure in the fuel connection between the first and second fuel pump after the engine is turned off. Shutting off the leakage line after the engine is turned off mainly prevents fuel from passing through the gap between the movable pump element and the boundary of the pump chamber of the second fuel pump and flowing back into the reservoir. This would lead to a gradual decrease of the pressure in the fuel connection upstream of the second fuel pump.
Maintaining the pressure after a hot engine is turned off prevents gas bubbles from forming in the connection between the first and second fuel pump. Such gas bubbles form when the fuel disposed in the fuel lines between the fuel pumps and is heated by thermal conduction from the engine. However, if the pressure is maintained even when the engine is turned off, as is possible with the fuel system according to the invention, then the formation of such gas bubbles can be prevented to a large extent, which considerably improves the starting behavior of an engine equipped with the fuel system according to the invention.
However, in order to keep the stress on the pressurized components of the fuel system to a minimum, the valve device in the leakage line also has a pressure relief function in addition to the shutoff function. After the hot engine is turned off, the heating of the fuel and the accompanying expansion of the fuel in the fuel line between the first and second fuel pump could cause an impermissible pressure increase in this region. Such an impermissible pressure increase is prevented by the pressure relief function of the valve device. The components in the fuel connection upstream of the high-pressure fuel pump are consequently protected from impermissibly high pressures even when the engine is turned off, which extends their service life. In addition, less expensive components designed for lower pressures can also be used.
The fuel system according to the invention consequently assures a favorable hot starting behavior of the correspondingly equipped engine; on the other hand, the fuel system is assured of being reliable and the stress on the pressurized components of the fuel system is kept to a minimum.
A first modification discloses that the same valve element is used for both functions in the valve device. A corresponding valve device is very small.
It is also particularly preferable that the shutoff function of the valve device can be electrically triggered. This makes it possible, when the motor control unit signals that the engine is turned off, for the shutoff function of the valve device to be activated by a simple control signal.
An easily manufactured, small embodiment of a valve device with a combined shutoff and pressure relief function is comprised in that the valve device has a valve element that is prestressed to perform the pressure relief function and can be electrically actuated counter to the prestressing force in order to disable the shutoff function.
It is particularly advantageous for the valve device to be situated in the vicinity of the engine, particularly in the vicinity of the second fuel pump. For example, it is conceivable to accommodate the valve device in the housing of the second fuel pump. Such a placement has the following advantage:
During operation of the internal combustion engine and therefore also during the operation of the second fuel pump, the shutting off of the leakage line is disabled. The leakage line is therefore largely unpressurized. Due to thermal conduction from the hot engine, the fuel in the leakage line is also heated up and vaporizes. Consequently, the leakage line contains only vaporous fuel at first after the engine is turned off.
If the shutoff function of the valve device is activated and the leakage line is closed when the engine is turned off, then situating the valve device far away from the second fuel pump would cause the closed system between the first fuel pump, the second fuel pump, and the valve device to contain a significant vaporous fuel volume at first. After cooling, fuel from the pump chamber can travel into this vaporous fuel volume, for example by means of a piston guidance gap of the second fuel pump (the gap between the piston and the housing), which can in turn lead to vapor formation in the pump chamber. However, if the valve device is situated as close as possible to the second fuel pump, then this vaporous fuel volume is only very small in any case and consequently cannot lead to any problems when the engine is restarted.
However, it is also possible for the valve device to be disposed in the vicinity of the reservoir. In this instance, the second fuel pump is provided with a bypass line that contains a throttle restriction and leads from the input of the second fuel pump to the leakage line. The cross section of the throttle restriction is selected so that during normal operation, the increase in the temperature of the reservoir is less than a limit value. This modification of the invention is based on the following concept:
Normally, the first fuel pump supplies the second fuel pump with a greater fuel quantity than is sent onward by the second fuel pump. In the current exemplary embodiment, this excess fuel is conveyed past the pump chamber and toward the beginning of the leakage line by means of the bypass line, which is contained in the second fuel pump, e.g. preferably in the housing wall. Consequently, during normal operation of the engine, in which the shutoff function of the valve device in the leakage line is in fact deactivated, a constant flushing flow is conveyed through the leakage line. This prevents fuel from remaining for a longer time in the leakage line and being heated by the leakage line so that it vaporizes.
Thus from the start, this modification according to the invention prevents vapor bubbles from forming in the leakage line. The fuel conveyed past the pump chamber can also be used to cool the second fuel pump, which further improves the hot operation of the fuel system and the engine equipped with it. However, care must be taken that the fuel heated during the cooling process in the second fuel pump does not cause an impermissible increase in the temperature of the fuel in the reservoir. This is assured through an appropriate design of the throttle restriction.
The invention also relates to a method for operating the fuel system of the type mentioned above. The valve device provided functions optimally when the shutoff function of the valve device is activated immediately after the engine is turned off and is deactivated immediately after the engine is started. The activation of the shutoff function of the valve device causes the valve device to close, whereas the deactivation of the shutoff function causes the valve device to open. With an electric actuation of the valve device, the shutoff function of the valve device is preferably activated when it is without current, whereas it is deactivated when supplied with current.
In a particularly preferable modification of this method, the first fuel pump continues to operate for a limited time after the engine is turned off. This ensures that the pressure in the associated region of the fuel system corresponds to the maximal pressure predetermined by the opening pressure of the pressure relief function of the valve device.
The increase of the pressure in the vicinity upstream of the second fuel pump, however, is only necessary when the engine is turned off when hot. It is therefore particularly preferable if the parameters relevant for a hot start of the engine are recorded and the first fuel pump and/or the valve device are triggered as a function of the recorded parameters.
It is particularly preferable if the parameters include a cooling water temperature and/or an intake air temperature and/or a speed and/or a load.
The pressure at the input of the second fuel pump can be adjusted in a particularly simple fashion by means of the speed of the first fuel pump.
The invention also relates to a computer program, which is suitable for executing the method mentioned above, when it is run on a computer. It is particularly preferable if the computer program is stored in a memory, in particular a flash memory.
The invention also relates to a control and/or regulating unit for controlling the fuel system described above; it is preferable if the control and/or regulating unit is provided with a computer program of the type described above.